From U.S. Pat. No. 2,143,130 a bone conduction hearing aid device is known, wherein a set screw with a shoulder portion engaging the opposite surface of a core cross piece is provided, so that by turning the set screw a pole piece core is deformed and withdrawn from its contact with the armature and retained in a fixed position in which a small effective magnetic gap of a fraction of a thousandth of an inch is maintained between the pole faces of the pole pieces and the armature during operation of the receiver.
Constructions effecting similar adjustment of the air gap are found in prior art documents GB444175; U.S. Pat. No. 2,127,468; U.S. Pat. No. 1,356,156 and U.S. Pat. No. 2,680,157.
In a further prior art example, the air gap is adjusted through grinding of a distance element as is explained in the following.